The limited supply and increasing cost of crude oil has prompted the search for alternative processes for producing hydrocarbon products. One such process is the conversion of oxygen-containing (for example methanol), halogenide-containing or sulphur-containing organic compounds to hydrocarbons, in particular, to light olefins, i.e. C2 to C4 olefins, or gasoline and aromatics. In the present application the conversion of said oxygen-containing (also referred to as oxygenates), halogenide-containing or sulphur-containing organic compounds to hydrocarbons, especially light olefins, is referred to as the XTO process. The interest in the XTO process is based on the fact that feedstocks, especially methanol can be obtained from coal, biomass, hydrocarbon residues, petcoke, organic waste or natural gas by the production of synthesis gas, which is then further processed to produce methanol. The XTO process can be combined with an OCP (olefin cracking process) process to increase production of olefins. The XTO process produces light olefins such as ethylene and propylene, as well as heavy hydrocarbons such as butenes and above. These heavy hydrocarbons are cracked in an OCP process to give mainly ethylene and propylene.
In accordance with U.S. Pat. No. 5,573,990 methanol and/or dimethylether is converted to light olefins in the presence of a catalyst, which contains at least 0.7% by weight of phosphorus and at least 0.97% by weight of rare earth elements, which are incorporated within the structure of the catalyst and allegedly enhance the hydrothermal stability of the zeolite. The rare earth elements are preferably rich in lanthanum, the content of lanthanum in the catalyst being preferably comprised between 2.5 and 3.5% by weight of the catalyst. The rare earth elements are introduced via impregnation onto the crystals in an aqueous solution of a lanthanum salt, for example La(NO3)3, or of mixed rare earth salts rich in lanthanum. The zeolite ZSM-5 based catalyst presents a mole ratio SiO2/Al2O3 comprised between 40 and 80, a crystal size comprised between 1 and 10 μm and adsorption capacities of n-hexane and water of from 10 to 11% by weight and of from 6 to 7% by weight respectively. Said ZSM-5 is synthesized in the presence of a template, then extruded with colloidal silica and converted to the hydrogen form by ion exchange using hydrochloric acid.
US 20060144759 A1 is related to the production of ethylene and propylene from the catalytic cracking of hydrocarbons, which may include an unsaturated bond, but no mention is made of oxygen-containing feedstocks. The aim was to find a catalyst, which could be used in a reactor permitting continuous regeneration of the catalyst. The zeolite thus cited as suitable is a high silica zeolite, preferably a ZSM-5 and/or a ZSM-11, having a SiO2/Al2O3 molar ratio ranging from 25 to 800 and carrying a rare earth element preferably chosen from lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium and dysprosium. It is stated that mere physical mixing of the zeolite with the rare earth compound is not sufficient. The zeolite may also contain other components such as an alkali metal, an alkaline earth metal, a transition metal, a noble metal, a halogen and phosphorus.
In accordance with US 2007/0032379 A1, an alkaline earth metal-containing MFI zeolite is disclosed, having a Si/Al atomic ratio of from 30 to 400, an alkaline earth metal/Al atomic ratio ranging from 0.75 to 15, and an average particle diameter ranging from 0.05 to 2 μm. This zeolite is selective for the production of lower hydrocarbons, e.g. ethylene and propylene, from dimethyl ether and/or methanol and is stated to have an extended catalyst life. The zeolite is obtained by synthesising a zeolite raw material solution, which contains a SiO2 source, a metal oxide source, an alkali source and a structure-directing agent, i.e. a template, in the presence of an alkaline earth metal salt, such as calcium acetate, and a zeolite seed crystal. This implies that the metal salt is present within the zeolite crystal structure.
According to U.S. Pat. No. 4,049,573, a catalytic process is provided for converting lower monohydric alcohols to a hydrocarbon mixture rich in ethylene and propylene and mononuclear aromatics with a high selectivity for para-xylene, using a catalyst comprising a crystalline aluminosilicate zeolite having a silica to alumina ratio of at least about 12, a constraint index in the range of 1 to 12, said catalyst having been modified by the addition thereto of a minor proportion of an oxide of boron or magnesium either alone or in combination, optionally with an oxide of phosphorus. The zeolite can be ion-exchanged to form metal-modified zeolites for example with nickel, zinc, calcium or rare earth metals. There is no mention of adding metal silicates to the zeolite.
In accordance with U.S. Pat. No. 3,911,041, methanol or dimethyl ether is subjected to conversion, at a temperature of at least about 300° C., with a catalyst comprising a crystalline aluminosilicate zeolite having a silica to alumina ratio of at least about 12, a constraint index of about 1 to 12, and containing phosphorus incorporated within the crystal structure thereof in an amount of at least about 0.78 percent by weight, preferably not higher than about 4.5 percent by weight. The zeolite, preferably, also has a dried crystal density of not less than about 1.6 grams per cubic centimeter. The crystalline aluminosilicate zeolite is first converted to the hydrogen form, then phosphorus is introduced by reaction with a phosphorus-containing compound having a covalent or ionic constituent capable of reacting or exchanging with a hydrogen ion. Thereafter, the phosphorus-modified zeolite is heated. There is no steaming of the zeolite prior to introduction of phosphorus. Preferably, prior to reacting the zeolite with the phosphorus-containing compound, the zeolite is dried, preferably in the presence of air and at an elevated temperature. The phosphorus-containing zeolite thus obtained may be further modified by impregnating the zeolite with zinc. This can be carried out by contacting the zeolite with a solution of a zinc salt, so that the zinc salt can fill the pore volume of the phosphorus-containing zeolite. Zinc-impregnated phosphorus-containing zeolites are claimed to have higher levels of conversion than those zeolites not impregnated with zinc.
Sano et. al. (Applied Catalysis, 33 (1987) 209-217) discusses the differences of Ca—H-ZSM-5, CaCO3/Ca—H-ZSM-5 and CaO/Ca—H-ZSM-5. The Ca—H-ZSM-5 zeolite was obtained by mixing aluminium nitrate, colloidal silica and calcium acetate, template and sodium hydroxide in solution. Thus, the calcium is contained within the crystal structure of the zeolite. After crystallisation of the zeolite from the hydrogel, the crystals were filtered off and then washed, dried, calcined at 500° C. for 16 hours, protonated and calcined again at 500° C. for 6 hours to obtain CaCO3/Ca—H-ZSM-5. To obtain CaO/Ca—H-ZSM-5, the CaCO3-containing catalyst was calcined once more for a further 24 hours at 600° C. The catalyst stabilities and long-term aging of Ca—H-ZSM-5, CaCO3/Ca—H-ZSM-5 and CaO/Ca—H-ZSM-5 were then compared in methanol conversions. Very slow decays of conversion and selectivity were observed for the CaCO3/Ca—H-ZSM-5 and the CaO/Ca—H-ZSM-5 zeolites. However Ca—H-ZSM-5 decayed rapidly, which is claimed to be due to the increased coke deposition on the catalyst surface. The amount of coke deposited on the CaCO3/Ca—H-ZSM-5 and the CaO/Ca—H-ZSM-5 zeolites was far less. On the other hand, the modification of the calcium-containing catalyst to a CaCO3— or CaO-containing catalyst did not seem to affect resistance to steaming. Thus, the extended catalyst life was attributed to the improved resistance to coking and not to the improved resistance to hydrothermal treatment.
EP448000 relates to a process for the conversion of methanol or dimethylether into light olefins in the presence of water vapour over a silicoaluminate of the pentasil structure having a Si/Al ratio of at least 10, thereby producing at least 5 wt % of ethylene, at least 35 wt % of propylene and at most 30 wt % butenes by (1) using a total pressure of 10 to 90 kPa, (2) a weight ratio of water to methanol of 0.1 to 1.5, (3) a reactor temperature of 280 to 570° C. and (4) a proton-containing catalyst of the pentasil-type, having an alkali-content of at most 380 ppm, less than 0.1 wt % of ZnO and less than 0.1 wt % of CdO and a BET surface area of 300 to 600 m2/gram and a pore volume of 0.3 to 0.8 cm3/gram.
WO2007/043741 discloses a catalyst for producing light olefins from a hydrocarbon feedstock wherein the catalyst consists of a product obtained by the evaporation of water from a raw material mixture comprising 100 parts by weight of a molecular sieve with a framework of Si—OH—Al groups, 0.01-5.0 parts by weight of a water-insoluble metal salt, and 0.05-17.0 parts by weight of a phosphate compound. The water-insoluble metal salt is a metal salt with a solubility product (Ksp) of less than 10−4, i.e. a pKsp of more than 4. This includes oxides, hydroxides, carbonates or oxalates of metals with an oxidation state of more than +2, preferably alkaline earth metals (Mg, Ca, Sr, and Ba), transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) and heavy metals (B, Al, Ga, In, Ti, Sn, Pb, Sb and Bi). Metal silicates are however not disclosed. There is also no indication that this catalyst can be used in XTO processes.
Molecular sieves in combination with matrix and binder components for XTO and OCP processes are known in the arts. Usually, the binder and matrix are chemically neutral materials, typically serving only to provide desired physical characteristics to the catalyst composition. Usually, they have very little effect on catalytic performance. These molecular sieve catalyst compositions are formed by combining the molecular sieve and the matrix e.g. an inorganic oxide such as alumina, titania, zirconia, silica or silica-alumina with a binder, e. g. clay, to form a coherent, stable, attrition-resistant composite of the sieve, matrix material and binder. In particular, the use of silica (SiO2) as a binder/matrix material is well known in the art. This solid is neutral and is selected when catalytic effects of the binder/matrix are undesired. Metal modified zeolites, particularly, P-zeolites and their use as XTO catalysts are known in the art. Typically, rare earth elements, which are very expensive, are used in such catalyst composites.
Metal is introduced typically in the form of water-soluble metal salts, mainly oxides/oxide-precursor salts, by ion-exchanged/impregnation. However, ion-exchange/impregnation potentially leads to the modification of the acidity of catalytic sites throughout the whole microporous structure of the molecular sieve. This could lead to decreased catalytic activity. Metal oxides are chemically active compounds. Without taking special precautions during pre-treatment and catalyst formulation these compounds may partially damage the molecular sieve pore structure. The proposed current invention is very different from the prior art. The combination of molecular sieves with chemically inert metal silicates allows selectively modifying only the sites located on the external surface and in the pore mouths of the molecular sieve. As a result, the formation of side products is avoided and coke formation is decreased without losses in the catalyst's activity by minimising reactions outside the microporous space.
A huge variety of naturally occurring and synthetically produced silicates are known in the art.
For example, U.S. Pat. No. 3,729,429 discloses layered complex metal silicate compositions, in particular chrysotiles, and their preparation.
U.S. Pat. No. 3,806,585 discloses the production of a hydrous calcium silicate composed preponderantly of xonotlite in the shape of rod crystals, which is described as having outstanding refractory properties, whereby moulded bodies comprised primarily of xonotlite provide strength unattained by other inorganic materials. The specification discloses that hydrous calcium silicate of the xonotlite type has use in construction as a fire proof coating material, as a fire proof moisture retaining material and as a potential filler for plastics and rubber products.
U.S. Pat. No. 3,804,652 discloses a method of producing calcium silicate-based products, such as drain pipes and insulating material.
U.S. Pat. No. 3,928,539 discloses a method of producing hydrous calcium silicates such as xonotlite, tobermorite and the like.
U.S. Pat. No. 3,915,725 discloses a process for producing hollow spherical aggregates of xonotlite, which are employed to form shaped articles.
U.S. Pat. No. 4,298,386 discloses the production of globular secondary particles of the woolastonite group of calcium silicate crystals, including woolastonite and xonotlite.
U.S. Pat. No. 7,294,604 discloses the use of calcium silicate as a catalyst support for metal supported hydrogenation and dehydrogenation catalysts.
Thus, the current invention proposes an improved catalyst for XTO and/or OCP processes.
It is thus an aim of the invention is to find a catalyst for XTO and/or OCP processes with an increased yield of light olefins.
It is another aim of the invention to find a catalyst for XTO and/or OCP processes with a higher stability.
In addition, it is another aim of the invention to find a catalyst for XTO and/or OCP processes with reduced selectivity for paraffins.
It is an aim of this invention to avoid the use of heavy and expensive rare earth metals.
The invention fulfils at least one of the above aims.